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VOL. 69 (4) ADENOSINE RECEPTORS

for modulation of synaptic transmission, might originate not only from
nerve terminals but also from the activated postsynaptic component (see
references in Cunha; 2001a, Dunwiddie and Masino; 2001, Latini and
Pedata; 2001) as well as from surrounding non-neuronal cells. Both
astrocytes and dendritic compartments are apparently devoid of a fast
exocytotic apparatus and yet they are also able to release ATP (Caciagli,
Ciccarelli, Di Iorio, Ballerini and Tacconelli; 1988) by a mechanism
largely unknown (Bodas, Aleu, Pujol, Martin-Satué, Marsal and Solsona;
2000, Bodin and Burnstock; 2001).

Thus, there are several mechanisms that contribute to the
formation of extracellular adenosine. And most importantly, it should be
stressed that to understand how adenosine is released, one needs to keep
in mind that different stimuli will trigger different ways of generating
extracellular adenosine. Figure 1 schematically represents the possible
contributions of the different compartments in the CNS to control the
extracellular levels of purines.

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